It's shown in the flashback episode that Amon was the stronger bender among the brothers.
Episode 11: Skeletons in the closet
Tarrlok: A few years later, my father taught us to bloodbend anytime, without the need of the full moon. We practiced constantly, and I hated every minute of it. I had no stomach for manipulating helpless animals. My brother, however, seemed to revel in this newfound power. He was a prodigy, mastering my father's psychic bloodbending technique by the time he was fourteen.